The manufacturing of semiconductor devices includes the deposition of metal films as well as a multitude of other materials. To optimize the production process and control the performance of the devices, properties of the metal films and other materials must be carefully monitored. Of course, it is desirable to minimize cost associated with the manufacturing process. To that end, metrology tools should be as precise and as accurate as necessary, be economical, be reliable and have an adequate throughput.
One parameter of deposited metal films that is closely monitored is the film thickness. Another parameter of interest is the sheet resistance. The measured resistance is important for analyzing the performance of the electronic devices in, e.g., an integrated circuit, while the thickness of the film is indicative of how well the device is made with respect to the entire process flow. The accurate measurement of sheet resistance and thickness of thin films is important in industries such as semiconductors and magnetic head fabrication.
A metrology tool that is sometimes used to measure conductive film thickness is an x-ray tool. Unfortunately, X-ray tools are not capable of measuring the sheet resistance. An eddy current sensor is commonly used to measure the thin film sheet resistance associated with conductive samples. While the standard eddy current sensor cannot measure the actual film thickness, the measured sheet resistance can be converted into film thickness with knowledge of the resistivity of the material. Additionally, eddy current measurements can be employed to characterize the magnetic properties of samples or detect the presence of defects or voids in conductive samples.
One factor limiting the precision and accuracy of an eddy current measurement is the change in resistance of the conductive film as a function of temperature. With a well designed system, the change in resistance as a function of temperature may be the largest source of error. Unfortunately, it is often impractical, inconvenient or time consuming to measure or control the temperature of the sample close to or simultaneously with the eddy current measurement. Attempting to control the temperature of the sample is undesirable as it requires additional hardware and unwanted expenses, complicates the robotics, and decreases throughput if the sample must achieve temperature stability. Further, in some applications, it is desirable to minimize contact with the sample (for contamination reasons) disallowing direct contact of the sample with a large thermal heat sink (a temperature controlled mass). Another option is to attempt to measure the temperature of the sample at the time of measurement. However, non-contact temperature measurement means (such as infrared sensors near room temperature) typically do not have the accuracy that is required. Further, temperature measurement methods relying on contacting the sample may be prohibited due to potential damage or contamination or they may complicate the robotics.
Thus, what is needed is an improved resistance and thickness measurement tool that reduces or eliminates errors caused by resistance changes caused by temperature.